$re = '/[Xx]yz(?=[^<>]*<)(?!(<(?:a|span)>(?:.|\n))*</(?:span|a)>)/im';
$str = 'xyz
Company name nissan and Xyz abc product.
<p>Company name xyz and xyz abc product.</p>
<a href="xyz"><span>Xyz</span></a>
<a href="Xyz"><span>xyz</span></a>
<span>xyz</span>
<p>You know Nissan become now <a href="xyz.com"><span>xyz</span></a> Micra.</p>
You know xyz become now http://www.xyz.com.
<p>Now .xyz. become now www.xyz.com</p>
<p>www.xyz.com</p>
<p>Hello you know Xyz is Now xyz become now www.xyz.com</p>
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php